Method for recovering purified sodium bicarbonate and ammonium sulfate

ABSTRACT

A process for recovering sodium bicarbonate and ammonium sulfate from a sodium sulfate solution. The sodium sulfate solution can be pure or contain other compounds such as sodium sulfite, carbonate, chloride, fluoride, nitrate and nitrite as would be the case if the sodium sulfate solution were derived from a sodium bicarbonate flue gas purification process. Carbon dioxide and ammonia gases or solid ammonium bicarbonate are added to the sodium sulfate solution to precipitate sodium bicarbonate which is removed from solution. The remaining solution is treated in a unique series of precipitation steps in which reactants are first recycled back to the sodium bicarbonate crystallizer and then the amount of sodium in the solution is adjusted to an amount that allows high grade ammonium sulfate fertilizer product to be produced. The process is accomplished using evaporation and precipitation unit operations in a unique sequence that results in 100% conversion of the sodium salt feed stock to sodium bicarbonate and ammonium sulfate in a commercially viable manner.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is the first application filed for the present invention.

FIELD OF THE INVENTION

[0002] The present invention relates to a method for recovering purifiedsodium bicarbonate and ammonium sulfate from a solution containingprimarily sodium sulfate. More particularly, the present inventionrelates to a method of obtaining sodium bicarbonate and ammonium sulfatefrom a solution containing primarily sodium sulfate using evaporationand precipitation unit operations in a unique sequence that results innearly 100% recovery of the feed stock in a commercially viable manner.

BACKGROUND OF THE INVENTION

[0003] The preparation of sodium bicarbonate and ammonium sulfate hasbeen discussed at length in the prior art. One of the most recentpatents regarding this technology is U.S. Pat. No. 6,106,796, issued toPhinney, Aug. 22, 2000. This patent effectively demonstrates the factthat in all of the prior art, the ability to produce a non contaminatedammonium sulfate product does not exist. This patent is effective forsynthesizing high quality ammonium sulfate and sodium bicarbonate byprogressive precipitation. This sequencing of precipitations results in“partitioned decontamination” by continuously removing contaminationfrom a predecessor solution which has already been exposed topurification.

[0004] Canadian Patent No. 2,032,627, issued Jan. 14, 1997 to Thompsonet. al., teaches yet another process for producing sodium carbonate andammonium sulfate from naturally occurring sodium sulfate. The referenceis concerned with the preparation of a double salt of sodium andammonium sulfate. This is a source of contamination when one is tryingto form reasonably pure ammonium sulfate and the presence of any doublesalt and sodium in an ammonium sulfate product does nothing other thanreduce the value of the ammonium sulfate to a non-commercial product. Inthe methodology, it is clearly stated on page 13, beginning at line 8:

[0005] “ . . . the brine remaining after screening off the solid sodiumbicarbonate contains a mixture of unreacted sodium sulfate, ammoniumsulfate, ammonium bicarbonate and minor amounts of sodium bicarbonate.This brine is transferred by a pump 36 into a gas recovery boiler 31where it is heated to a temperature of 95° to 100° C. Under theseconditions, the ammonium bicarbonate breaks down and sodium bicarbonatedissolved in the brine reacts with ammonium sulfate to produce sodiumsulfate, carbon dioxide and ammonia. Carbon dioxide and ammoniadissolved in the brine boil off, leaving in the solution a mixturecomposed mostly of sodium and ammonium sulfate The carbon dioxide andammonia so regenerated are cooled in a gas cooler 32 and returned to thereactor 21 by a blower 33 after being further cooled in a gas cooler 34.This regeneration step minimizes the amount of carbon dioxide andammonia used in the process.”

[0006] It is clear that the brine is evaporated and that the ammoniumsulfate is reacted with the brine to produce sodium sulfate inter alia.The phase equilibria relationship between the elements present in thesystem was not recognized.

[0007] The teachings of this reference provide for a closed loop systemfor a sodium sulfate and ammonium sulfate saturated solution system.This system results in the formation of double salt. The teachings arelimited in that it was believed that the solubility difference couldyield an ammonium sulfate product. This is incorrect; the result is anammonium sulfate contaminated system.

[0008] In Stiers et al, U.S. Pat. No. 3,493,329, the teachings aredirected to the preparation of sodium bicarbonate and hydrochloric acid.This goal is consistent with the teachings of Stiers et al. at column 11of the disclosure beginning at line 23 through line 43, wherein thefollowing is indicated:

[0009] “If, instead of precipitating the double salt in the first stageof the process, it is preferred to precipitate ammonium sulfate, thefollowing procedure may be adopted.

[0010] Referring now to FIG. 10, it will be seen that each of the threecurves which divide this figure into three parts corresponds to thesimultaneous precipitation of two salts.

[0011] At any given temperature, the point representing a system may bevertically displaced by removing some of the water from the solution. Inorder to precipitate ammonium sulfate instead of the double salt, it isnecessary to operate at a temperature greater than that at the triplepoint, i.e., about 59° C.

[0012] The point A, which corresponds to about 63° C. is suitable, sinceit is sufficiently distant from the triple point to avoid unwantedprecipitation of the double salt without requiring too much heat.

[0013] It is clear that at the point A, there is simultaneousprecipitation of sodium sulfate and ammonium sulfate, but this is in theform of a mixture of the two salts rather than as a double salt.”

[0014] The teachings of the Stiers et al. reference not only areinsufficient to direct one to formulate ammonium sulfate in a purity ofgreater than 75%, but the disclosure is further completely absent of anyteaching on how to obtain ammonium sulfate singly. The Stiers et al.reference does not and can not result in the generation of ammoniumsulfate as a single product as is clearly possible by the teachings ofthe present invention.

[0015] By following the Stiers et al. methodology, one cannot generate apure ammonium sulfate product, since the reference does not recognizethe limitations of the phase equilibria of the salt system and thecombination of steps necessary to overcome the inherent contaminatingsteps associated with this salt system. Although there is a reference topoint A in FIG. 10 of Stiers et al. for the preparation of the product,it is clear that although no double salt is indicated to be in themixture, there is no indication that the product does not include mixedsalt. This is reflected in the disclosure where Stiers et al. indicatesthat there is simultaneous precipitation of sodium sulfate and ammoniumsulfate. This is consistent with the data that Stiers et al. provides asindicated at column 12 beginning at line 21. There is no data presentedwhere the quantity of ammonium sulfate, standing on its own, is setforth. In each case, the data presented is expressed as a proportionprecipitated in a compound, i.e, combined salt inter alia. Finally, fromthe text set forth beginning at line 32, Stiers et al. indicates that:

[0016] “ . . . From the foregoing it will be seen that the processaccording to the invention may be carried out by precipitating theammonium sulfate in the form of the double salt, or as (NH₄)₂SO₄simultaneously with sodium sulfate, or by precipitating itsimultaneously in the form of ammonium sulfate and in the form of thedouble salt.”

[0017] From a review of FIGS. 10 and 11 (in Stiers et. al.), the factthat no ammonium sulfate is generated singly becomes evident. No data ispresented for ammonium sulfate generation; the results from practicingthis methodology are only a mixed salt and a double salt. Nothing elseis obtainable by practicing this method.

[0018] Finally, Kresnyak et al. in U.S. Pat. No. 5,830,442, issued Nov.3, 1998, teach an improved process for producing ammonium sulfate. Thisprocess is attractive where energy consumption and conversion efficiencyare not of primary concern. In this process, sodium sulfate is removedby significant energy input to the evaporators with subsequent cooling.The result is a 2:1 ratio of double salt to solution which then must beevaporated in order to recover ammonium sulfate.

[0019] In view of the limitations of the prior art, it is evident that aneed remains for a process whereby ammonium sulfate and sodiumbicarbonate can be formulated in high yield at a high purity usingcommercially viable, energy efficient unit operations in the propersequence. The present invention fulfils these objectives in an elegantmanner.

SUMMARY OF THE INVENTION

[0020] One object of the present invention is to provide an improvedprocess for making ammonium sulfate and sodium bicarbonate from asolution of sodium sulfate and other minor sodium salts such as sodiumsulfite, carbonate, chloride, fluoride nitrate and nitrite.

[0021] The sodium bicarbonate produced is suitable for use as ascrubbing agent for flue gas purification. In the event that food gradesodium bicarbonate is desired, the same may be washed in order toachieve United States Pharmacopoeia standards.

[0022] A further object of one embodiment of the present invention is toprovide a method for recovering purified sodium bicarbonate and ammoniumsulfate from a solution, containing sodium sulfate, comprising the stepsof:

[0023] A) providing a solution containing sodium sulfate;

[0024] B) precipitating, in a single precipitation step, sodiumbicarbonate precipitate to reduce the sodium bicarbonate concentrationin solution, the solution containing ammonium sulfate, the singleprecipitation is accomplished bystep including;

[0025] C) removing the the sodium bicarbonate precipitate out ofsolution;

[0026] D) converting in a conversion step, reactants from step B) tosodium bicarbonate and conversion step including

[0027] i) adding combined salt containing ammonium bicarbonate andGlauber's salt to inlet sodium sulfate solution;

[0028] ii) adding carbon dioxide and ammonia gas to the the inlet sodiumsulfate solution;

[0029] iii) maintaining a ammonium to sodium ratio of not less than 1;

[0030] iv) operating at a temperature sufficient to prevent excessivegas production; and

[0031] v) removing the sodium bicarbonate precipitate out of solution;

[0032] E) mixing the solution from step B) with an ammoniumsulfate/sodium sulfate double salt;

[0033] F) cooling the mixture from step E) to form a combined salt;

[0034] G) precipitating the combined salt and removing the combined saltout of solution;

[0035] H) removing residual bicarbonate from the solution from step G);

[0036] I) mixing the solution from step H) with mother liquor;

[0037] J) cooling the mixture from step I) to precipitate double salt;

[0038] K) separating precipitated double salt from the solution andrecycling to step E); and

[0039] L) recovering ammonium sulfate from the solution of step K) byconcentrating the solution.

[0040] In terms of the acidification, any suitable acid may be used toremove residual bicarbonate and/or carbonate compounds. This results inthe liberation of carbon dioxide gas which then may be recycled into thesodium bicarbonate precipitation step. An acid useful to achieve thisgoal is sulfuric and it will be appreciated by those skilled in the artthat the sulphuric acid employed will be relatively high molarity and ofsimilar ionic composition to the solution being altered.

[0041] The unit operations and sequencing as set forth herein providefor nearly 100% conversion of sodium sulfate and ammonium bicarbonate toammonium sulfate and sodium bicarbonate in a commercially viable manner.

[0042] Having thus described the invention, reference will now be madeto the accompanying drawings illustrating preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043]FIG. 1 is a process flow diagram according to one embodiment ofthe present invention; and

[0044]FIG. 1A is a process flow diagram of a further embodiment of thepresent invention; and

[0045]FIG. 2 is a process flow diagram of yet another embodiment of thepresent invention; and

[0046]FIG. 2A is a process flow diagram of a another embodiment of theprocess of the present invention;

[0047]FIG. 3 is a further process flow diagram of one embodiment of theprocess of the present invention;

[0048]FIGS. 4 and 5 are Janecke diagrams that represent the chemicalequilibrium involved in the sodium bicarbonate precipitation step;

[0049]FIG. 6 is a Janecke diagram that represents the chemicalequilibrium involved in the combined salt precipitation step;

[0050]FIG. 7 is a T-X (temperature-composition) phase diagram thatrepresents the chemical equilibrium involved in the production of highquality ammonium sulfate from a solution containing ammonium, sulfateand sodium ions.

[0051]FIG. 8 is a process flow diagram according to the prior art;

[0052]FIG. 9 is another process flow diagram according to the prior art;

[0053]FIG. 10 is a further process flow diagram according to the priorart;

[0054] Similar numerals in the figures denote similar elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0055] In FIG. 1, the overall process in accordance with a firstembodiment is illustrated and globally referenced as numeral 10. As afirst step, which is optional, the pure or contaminated solution may bepretreated at 11 to remove sodium metals which may be present such assodium fluoride, chloride, etc. The sodium sulfate solution may beconcentrated In the process flow diagram shown, a simple evaporationdrives off moisture and thus increases the concentration in solution.Any suitable means may be used to achieve this function.

[0056] It has been found to be desirable to concentrate the inletsolution such that the feed to the sodium bicarbonate crystallizer(including all recycles) is saturated or nearly saturated in order tomaximize the once through conversion of the sodium sulfate and ammoniumbicarbonate or carbon dioxide and ammonia feeds to the crystallizer.This detail is important in minimization of the size of the recyclestreams required to achieve 100% conversion of the inlets. As oneskilled in the art will realize, minimization of recycle stream sizeminimizes energy consumption. This fact was not recognized in the priorart and when combined with the previously unrecognized need to ensurethe optimal ammonium to sodium ratio in the sodium bicarbonatecrystallizer, can increase the once through conversion of the reactantsto sodium bicarbonate from as low as 30% to as high as 65%. By takingcare to maximize the once through conversion, the energy consumption ofthe process can be reduced by a factor of 10.

[0057] It has been found that the optimal ammonium to sodium feed ratioto the sodium bicarbonate precipitation step is that which results in aslight excess of ammonium (ratio of between 1.01 and 1.10). Although ona once through basis, an ammonium to sodium ratio of 0.912 results inthe greatest once through conversion to sodium bicarbonate, the largerecycle stream sizes that result from the excess sodium rapidlydeteriorate process economic viability.

[0058] The preparation of the sodium sulfate will occur in vessel 12 andonce prepared, the solution is then transferred to a precipitator 14 forprecipitating sodium bicarbonate. This precipitation is accomplished bythe addition of carbon dioxide and ammonia gas or solid ammoniumbicarbonate together with combined salt (ammonium bicarbonate andGlauber's salt derived from a further unit operation discussedhereinafter) in the correct combination to achieve the previouslydiscussed optimal ammonium to sodium ratio. In the further combined saltprecipitation step (vessel 18), double salt contamination (containingammonium sulfate product) in the combined salt precipitate will reducethe overall process efficiency by reducing the once through efficiencyof the sodium bicarbonate crystallizer. To one skilled in the art, theinclusion of double salt in the combined salt will pull the reactantpoint on the Janecke (see FIG. 4 and Example 1) towards the sodiumbicarbonate/ammonium bicarbonate solubility line which by the use of thelever rule will reduce the once through process efficiency. This pointwas overlooked in the prior art and is an important aspect in respect ofthe novelty of the present invention. In addition, it has been foundoptimal to maintain the temperature of the combined slurry in vessel 14in the optimal range for sodium bicarbonate precipitation of 35 to 40°C.

[0059] In an example to follow, it will be illustrated how theapplication of the chemical equilibrium involved in the sodiumbicarbonate precipitation step may be used to maximize the once throughconversion to sodium bicarbonate in vessel 14. The ability to maximizethe once through conversion in the sodium bicarbonate precipitatorallows one skilled in the art to optimize the economics of the inventionand ensure economic viability. The sodium bicarbonate precipitate andsolution are then separated in a separator 16 where the solid isseparated and comprises high purity sodium bicarbonate.

[0060] In terms of the liquid from separator 16, the same is then mixedwith an ammonium sulfate/sodium sulfate double salt derived from afurther unit operation and possibly some water and cooled (optimallybetween −2 C and 2 C) in vessel 18 resulting in the precipitation of anammonium bicarbonate/Glauber's salt combined salt. This combined saltprecipitation step to optimize and stabilizer the process is not part ofthe prior art. The temperature range of −2 to 2 C is optimal but itshould be apparent to one skilled in the art that a wider temperaturerange will work, although not as efficiently. The combined salt isseparated from the solution in separator 20. The combined salt is thenreintroduced into the sodium bicarbonate precipitation stage in vessel14 as the ions in the combined salt represent unused reactants and notproducts (ammonium sulfate). As one skilled in the art will recognize,the water and bicarbonate concentration in the combined saltprecipitation step are extremely important. If the water and bicarbonateconcentration are not correct, the sodium sulfate/ammonium sulfatedouble salt could also precipitate and contaminate the combined salt. Assuch, the correct amount of water and bicarbonate in the form of carbondioxide have to be added to this step to ensure the precipitation ofcombined salt only. The amount of water and carbon dioxide required isnot obvious. Possible sources of carbon dioxide to be added to vessel 18include the carbon dioxide produced in the sodium bicarbonateprecipitation step, the carbon dioxide derived from the furtherbicarbonate removal step or an external carbon dioxide source.

[0061] The precipitation and recycling of the combined salt is essentialto obtaining nearly 100% conversion of the sodium salt feed to sodiumbicarbonate in an economical manner. This fact was not recognized in theprior art. Without the combined salt precipitation step, all of theunconverted bicarbonate from vessel 14 would feed ahead to the bicarbremoval step (denoted as 22) and would have to be recovered and recycledas gaseous carbon dioxide. One skilled in the art will readily recognizethat it is far less energy intensive to recycle the unconvertedbicarbonate as a solid rather than a gas. In addition, if double saltcontaining ammonium sulfate (a process product) contaminates thecombined salt precipitate, then ammonium sulfate is unnecessarilyrecycled back to the beginning of the process resulting in furtherdeterioration of the viability of the process through increased energyconsumption due to reduced once through conversion.

[0062] The solution from vessel 18 is then treated by an acidificationoperation, broadly denoted by numeral 22 to remove residual bicarbonatefrom the solution. Removal of the residual bicarbonate is essential tothe production of pure ammonium sulfate fertilizer in a further unitoperation. The acidification may comprise any suitable acid treatment,however, one acid which has been found to be particularly useful issulfuric. Once the sulfuric acid contacts the solution, the carbonatesare liberated from the solution as carbon dioxide because of the pHdependent equilibrium between bicarbonate ion and aqueous carbondioxide. The carbon dioxide is then returned to vessel 14 or 18 via line24. The solution is then mixed with mother liquor derived from thedownstream ammonium sulfate concentration step (denoted as 32), passedto vessel 26 and cooled (optimally between −2 C and 2 C) to precipitatedouble salt. The subsequent solution and double salt solid are separatedby separator 28. The temperature range of −2 to 2 C is optimal but itshould be apparent to one skilled in the art that a wider temperaturerange will work, although not as efficiently.

[0063] The double salt is returned via line 30 to the combined saltprecipitator vessel 18. The precipitation of double salt in this step isessential to the ability of the invention to produce high qualityammonium sulfate fertilizer product. If the amount of sodium in thesolution feeding the ammonium sulfate precipitation step (step 32) isnot controlled (by precipitating and recycling the double salt, it isnot possible to precipitate high quality ammonium sulfate. Example 4 tofollow will illustrate the importance of understanding how the sodiumcontent of the solution feeding the ammonium sulfate precipitation stephas to be controlled to produce high quality ammonium sulfate. This wasnot previously appreciated in the prior art. FIG. 7 illustrates thechemical equilibrium involved.

[0064] The solution from separator 28 is exposed to a concentrationoperation, globally denoted by numeral 32, where the ammonium sulfatebearing solution is concentrated to cause ammonium sulfateprecipitation. This could be achieved by any known means such asstraight forward evaporation. The solution is then separated from thesolid by separator 34. The solid comprises high quality ammonium sulfatefertilizer wet cake which can then be washed and formulated into amarketable form. The solution is returned to the double saltprecipitator via line 36.

[0065] In the event that the inlet source of sodium sulfate was notderived from a pure sodium sulfate source, for example from a flue gaspurification process utilizing dry and/or wet sodium bicarbonatescrubbing, the same may contain nitrate compounds and other impuritiessuch as but not limited to sodium chloride, sodium fluoride, etc. Ifthese impurities are present they are purged from the system at 38. Thispurge does not degrade the economics of the process as the purge itselfis a valuable fertilizer product since it contains ammonium sulfate,ammonium nitrate and other ammonium salts in solution. The impurities(Cl, F, Na, etc.) will be in low enough concentrations when the inletsodium sulfate solution is derived from for example, a flue gas sourceto allow the stream to be sold as a fertilizer product.

[0066] As an alternative, FIG. 2 illustrates a different possibilitywith respect to the preparation of the sodium bicarbonate and ammoniumsulfate.

[0067] In FIG. 2, the solution from separator 20 is passed into abicarbonate stripping tower 40 which may be of the packed or tray type.The tower can be either a refluxed or non refluxed distillation tower.The carbon dioxide and ammonia gases (and water vapor) liberated fromsolution are recycled via line 44 to vessel 14 or vessel 18. As in theacidification option, the bottoms liquid from the stripper 40 is treatedin vessel 26 in order to precipitate the double salt. The rest of thecircuit follows the same series of unit operations as those that havebeen set forth in the discussion for FIG. 1.

[0068] In terms of temperature, the overheads from the stripping towershould be kept as low as possible although the process will work over alarge temperature range. As low a temperature as possible is idealbecause the lower the temperature, the less water carry over there iswith the carbon dioxide and ammonia gas. As one skilled in the art willrecognize, the minimization of water recycle in the process willminimize energy consumption and equipment size. The practical limit tothe minimization of the stripper overhead temperature and water carryover is the fact that if the temperature drops too far below 65° C.,solid ammonium bicarbonate or other ammonium/carbonate salts willprecipitate in the line.

[0069] In the prior art, the fact that it is far less energy intensiveto recycle un-reacted bicarbonate in the solid form (as ammoniumbicarbonate) compared to carbon dioxide gas was not recognized. In thepresent invention, this is overcome by locating the bicarbonate recoverystep (acidification or stripper) after the combined salt precipitationstep.

[0070] In terms of further alternatives, the bicarbonate removal step(stripper or acid addition) could be located downstream of the doublesalt crystallization step (vessel 26) (see FIGS. 1A and 2A). Wherepractical, this configuration would allow for reduced energyconsumption.

[0071] An additional further alternative would be to carry out thebicarb removal step simultaneously with the ammonium sulfate solutionconcentration step (denoted as 32). This would eliminate the need for aseparate acidification or stripping unit operation for the removal ofbicarb (see FIG. 3).

[0072] In respect of temperatures, the combined salt and double saltprecipitators have been indicated to optimally function in a range of−2° C. to 2° C. The sodium bicarbonate precipitation step has beenindicated to optimally function in the range of 35 to 40 C. To oneskilled in the art, it should be apparent that the present inventionwill work outside of these temperature ranges but at reduced efficiency.

[0073] With respect to the individual precipitators and equipmentchoice, this will depend upon the size of the circuit, desired output,daily quantity, among a host of other factors.

EXAMPLES Example 1 Determination of Optimal Ammonium to Sodium Ratio inthe Sodium Bicarbonate Precipitation Step

[0074] The following example illustrates how the complex phaseequilibrium chemistry involved in the present invention can be used todetermine the optimum ammonium to sodium ratio in the sodium bicarbonateprecipitation step. The understanding of the chemistry demonstrated bythis example is required for all unit operations within the process.

[0075] The following equilibrium reaction equations represent theprocess in the sodium bicarbonate precipitation step (using either solidammonium bicarbonate or carbon dioxide and ammonia gas):

Na₂SO₄+2NH₄HCO₃

2NaHCO₃+(NH₄)₂SO₄

Na₂SO₄+2NH₃+2CO₂+2H2O

2NaHCO₃+(NH₄)₂SO₄

[0076] In order to understand the complexity of the phase equilibriumbehavior described in this reaction, a graphical representation of thesystem is required. The reciprocal salt pair quaternary system describedin this reaction can be represented on an isothermal ‘space model’.However, these space models are difficult to use from an engineeringperspective and do not easily provide a way of understanding the systemas a complete process.

[0077] One simplification of the ‘space model’ is known as a Janeckediagram or projection. In a Janecke diagram, the salt and water curvesof the ‘space model’ are projected onto a two dimensional graph.

[0078] The Janecke diagram shown in FIG. 4 represents the phaseequilibrium in the sodium bicarbonate crystallizer at a temperature of35° C. The abscissa (X axis) is the charge fraction of bicarbonate ions(and aqueous carbon dioxide, carbonate ions (CO₃ ²⁻) and carbamate ions(NH₂COO⁻)) calculated as follows:

X=Mols HCO₃ ⁻/(Mols HCO₃ ⁻+(2×Mols SO₄ ²⁻))

[0079] The ordinate (Y axis) is the charge fraction of sodium ionscalculated as:

Y=Mols Na⁺/(Mols Na⁺+Mols NH4^(+*))

[0080] (*includes aqueous ammonia and carbamate ions)

[0081] The saturated water content in weight percent can be shown at thegrid intersections. However, for clarity this feature is not included inthis figure.

[0082] The enclosed areas on the graph represent precipitation areas ofthe salt indicated with the pure component composition represented ateach corner. In these areas, the solution is in equilibrium with thesolid salt indicated if the water concentration is low enough to resultin precipitation of the salt. The curves or mutual solubility lines onthe graph represent solutions in equilibrium with the two salts oneither side of the line. The intersection of two lines or curvesrepresents solutions in equilibrium with three salts and is known as aninvariant point.

[0083] The Janecke diagram in FIG. 4 was created using a UNIQUAC(Universal Quasi Chemical) computer model.

[0084] The small circles on the diagram represent measured data pointsfrom various published sources lending credibility to the computer modelused to generate the diagram.

[0085] A crystallizer feed (reactants) contains the following moles ofthe various ions and 760 g of water:

[0086] Na⁺ ions=4.219 mols (97.0 g−MW=23 g/mol)

[0087] NH₄ ⁺ ions=5.512 mols (99.2 g−MW=18 g/mol)

[0088] HCO₃ ⁻ ions=5.590 mols (341.0 g−MW=61 g/mol)

[0089] SO₄ ²⁻ ions=2.115 mols (203.0 g−MW=96 g/mol)

[0090] Total Ions=740.2 g

[0091] Water=760.0 g

[0092] Total Feed (Reactants)=1500.2 g

[0093] The cation charge fraction is:

X=4.219/(4.219+5.512)=0.43

[0094] The anion charge fraction is:

Y=5.590/(5.590+(2×2.115))=0.57

[0095] When this point is plotted on the Janecke diagram (see FIG. 4) itfalls on the sodium bicarbonate precipitation area. Therefore, the firstsolid to form will be sodium bicarbonate if the water content is lessthan 78 wt % as indicated by the grid points (not shown in the diagramfor clarity). In this example, the water content of the reactants is50.7 wt %. Therefore, sodium bicarbonate will precipitate. Point (1.0,1.0) in the right top corner represents the composition of the firstsolid which we know to be one hundred percent sodium bicarbonate. Thecomposition of the mother liquor will change along the dashed line drawnthrough points (0.43, 0.57) and (1.0, 1.0) until the mother liquor anionand cation charge fraction point and water concentration meet. The threepoints must form a straight line through the initial reactants pointtermed an operating line. This operating line represents a unitoperation in the process.

[0096] If the ammonium bicarbonate/sodium bicarbonate saturation line isreached before the mother liquor charge fraction point and waterconcentration meet, then ammonium bicarbonate will begin toco-precipitate. The composition of the mother liquor will then changealong the sodium bicarbonate/ammonium bicarbonate saturation linetowards the left. The composition of the solid will begin to changealong the secondary ‘Y’ axis (1,Y2), moving down from one hundredpercent sodium bicarbonate.

[0097] The final operating line and end point solid and mother liquorcan be found by trial and error utilizing the lever rule. The lever ruleis a way of calculating the proportions of each phase on a phasediagram. It is based on conservation of mass and can be provenmathematically. For this example, the lever rule demonstrates that therelative mass amounts of solid and mother liquor are inverselyproportional to the distance of the end point of each phase from theinitial reactants point.

[0098] In this example, the mother liquor and solid end points are atthe ends of the solid line drawn through the initial reactants point(see FIG. 4).

[0099] The preceding illustrates how to use the Janecke diagram. Thefollowing illustrates how the Janecke diagram can be used for processoptimization.

[0100] Any mixture of sodium sulfate and sodium bi-carbonate will resultin an initial starting point (reactants) that falls on the diagonal linedrawn between points (0,1) (100% sodium sulfate) and (1,0) (100%ammonium bicarbonate) (see dashed line in FIG. 5).

[0101] The goal is to precipitate sodium bicarbonate, so the compositionof the feed (reactants) has to be adjusted such that the plot of thereactants anion and cation charge fractions falls on the sodiumbi-carbonate saturation surface. The following illustrates how todetermine the optimal reactants starting point or ammonium to sodiumratio.

[0102] If the feed has an equi-molar ratio of ammonium to sodium (A/Sratio=1.0) then the plot of the reactants charge fractions falls on thepoint (0.5,0.5) (point A in FIG. 5). If the water content in the feed isadjusted such that precipitation stops just at the sodiumbicarbonate/ammonium bicarbonate saturate line, we know that theresultant mother liquor and solids fall at points B and C respectively.The mass of sodium bicarbonate produced can then be determined by usingthe lever rule.

[0103] Looking at the diagram, the observation can be made that a feedwith an excess of sodium up to the point where the end point motherliquor stops just short of the sodium bicarbonate/ammoniumbicarbonate/double salt (Na₂SO₄—(NH₄)₂SO₄—4H₂O) invariant point (pointE) will provide the maximum “once through” yield of sodium bicarbonate.This feed point is shown as ‘D’ on the diagram. With a feedcorresponding to point D (and water content adjusted such thatprecipitation stops when the end point mother liquor just reaches thetriple point), the ratio of the distances on the Janecke (lever rule)results in the maximum amount of sodium bicarbonate produced. Any feedwith more or less excess sodium will result in less sodium bicarbonateproduction. The following three examples will illustrate this point. Tosimplify the analysis, it is assumed in all cases that precipitationwill stop when the mother liquor reaches the sodium bicarbonate/ammoniumbicarbonate saturation line. Therefore, the solid produced will alwaysbe one hundred percent sodium bicarbonate.

[0104] Case 1: Ammonium to Sodium Molar Ratio=1.0

[0105] If a feed has 1 mol of Na+ and an ammonium to sodium ratio of1.0, then the feed composition is as follows:

[0106] Na⁺=1 mol (23.0 g)

[0107] NH4⁺=1 mol (18.0 g)

[0108] HCO₃ ⁻=1 mol (61.0 g)

[0109] SO₄ ²⁻=0.5 mol (48.0 g)

[0110] Using the Janecke and the lever rule, the mass of solid sodiumbicarbonate produced is 53.7 g. We know this solid is 100% sodiumbicarbonate. Therefore, Na⁺ and HCO₃ ⁻ conversion to sodium bicarbonateis 63.9%.

[0111] Case 2: Maximum Once Through Sodium Bicarbonate Production (A/SMolar Ratio=0.912)

[0112] As discussed, point D in FIG. 5 represents the feed that willresult in the maximum production of sodium bicarbonate on a once throughbasis. Point D has Janecke coordinates of (0.477,0.523). If a feed has 1mol of Na⁺ then it contains 0.5 mols of SO₄ ²⁻. The moles of NH₄ ⁺ andHCO₃ ⁻ are equal and can be found from:

0.523=Mols Na⁺/(Mols Na⁺+Mols NH4⁺)

or

0.477=Mols of HCO₃ ⁻/(Mols of HCO₃ ⁻+(2×Mols SO₄ ²⁻)

[0113] These give 0.912 moles of NH₄ ⁺ and HCO₃ ⁻ and an ammonium tosodium molar ratio of 0.912. Therefore, we have a feed with thefollowing composition:

[0114] Na⁺=1 mol (23.0 g)

[0115] NH₄ ⁺=0.912 mol (16.4 g)

[0116] HCO₃ ⁻=0.912 mol (55.6 g)

[0117] SO₄ ²⁻=0.5 mol (48.0 g)

[0118] Total=143.0 g

[0119] As before, using the Janecke and the lever rule, the mass ofsolid sodium bicarbonate produced is 55.6 g, sodium conversion is 66.2%and bicarbonate conversion is 72.6%. These are the highest conversionsof sodium and bicarbonate possible on a once through basis.

[0120] Case 3: Ammonium to Sodium Molar Ratio of 2.33

[0121] Point F in FIG. 5 represents a feed with a large excess ofammonium. If a feed has 1 mol of Na⁺ then it contains 0.5 moles of SO₄²⁻. The mols of HCO₃ ⁻ and NH₄ ⁺ are equal and can be found from:

0.3=Mols Na+/(Mols Na++Mols NH4+)

or

0.7=Mols of HCO3−/(Mols HCO3−+(2×Mols SO42−)

[0122] These give 2.33 moles of NH₄ ⁺, 2.33 moles of HCO₃ ⁻ and anammonium to sodium ratio of 2.33. Therefore, we have a feed with thefollowing composition:

[0123] Na⁺=1 mol (23.0 g)

[0124] NH₄ ⁺=2.33 mol (41.9 g)

[0125] HCO₃ ⁻=2.33 mol (142.1 g)

[0126] SO₄ ²⁻=0.5 mol (48 g)

[0127] Total=255.0 g

[0128] Again, using the Janecke and the lever rule, the mass of sodiumbicarbonate produced is 30.3 g, sodium conversion is 36.1% andbicarbonate is 15.5%.

[0129] These examples illustrate that on a once through basis, anammonium to sodium ratio of 0.912 results in the maximum once throughconversion of reactants to solid sodium bicarbonate. However, theseexamples do not show the magnitude of the combined salt and double saltrecycle streams that result from the different feed ammonium to sodiumratios. It has been found that a slight excess of ammonium is favorablebecause when there is even a slight excess of sodium, the recyclesbecome extremely large. This is because ammonia is very volatile incomparison to sodium which will reduce the final ammonium to sodiumratio to 1.0. Sodium is non-volatile and will stay in solution andbuild-up in the system. From an equipment capital cost and energyconsumption point of view, these large recycles would deteriorate theeconomics.

[0130] The determination of the fact that a slight excess of ammonium isfavorable was done utilizing a process simulator. To try and determinethis fact with hand calculations would be impractical due to the timerequired. A thorough understanding of the chemistry combined with theutilization of a powerful process simulator has enabled the optimumammonium to sodium ratio to be found. The process simulator Hysis™coupled with the OLI™ property package was used. It has been found thatHysis™ matches very closely to measured analytical data for all of thechemical equilibrium involved in the present invention. The followingtable illustrates how well Hysis™ matches published measured data (whichthe Janecke is based on) for the preceding examples. TABLE 1Determination of Optimal Ammonium To Sodium Ratio- Hysis vs JaneckeDiagram Janecke Hysis % Difference Example 1 (A/S Ratio = 1.0) SolidProduced (g) 53.7 53.1 1.1 Sodium Conversion (%) 63.9 63.2 1.1Bi-carbonate Conversion (%) 63.9 63.2 1.1 Example 2 (A/S Ratio = 0.912)Solid Produced (g) 55.6 53.8 3.2 Sodium Conversion (%) 66.2 64.0 3.2Bi-carbonate Conversion (%) 72.6 70.3 3.2 Example 3 (A/S Ratio = 2.33)Solid Produced (g) 30.3 27.0 11.1 Sodium Conversion (%) 36.1 32.1 11.1Bi-carbonate Conversion (%) 15.5 13.8 11.1

[0131] Therefore, because Hysis™ is known to match measured equilibriumdata applicable to the present invention, its results are used ratherthan hand calculations and Janecke diagrams for the remaining examples.

[0132] In addition to the use of a process simulator to model andunderstand the process, proprietary lab testing of the chemistryinvolved in the present invention was done. This testing providedadditional verification of the validity of the results of the simulatorand also showed that the chemical processes involved in the presentinvention are equilibrium based and are not limited kinetically. Thisfact is important. If the chemistry was kinetically limited, this woulddeteriorate the economic viability of the process. The following tableprovides a sample of how well Hysis matches the results of theproprietary lab testing. TABLE 2 Comparison of Hysis Results to Resultsof Proprietary Testing Proprietary Testing Hysis % Error 27.5 wt % SSFeed (g/hr) 950 950 n/a ABC Feed (g/hr) 530 530 n/a Glauber's Salt Feed(g/hr) 290 290 n/a Cryst. Temp © 40 40 n/a Centrate Ph 8.65 7.85 10.2Centrate Product (Note1) (g/hr) 1430 1445 −1.0 SBC Product (g/hr) 270275 −1.8

Example 2 Illustration of the Impact of Lower Sodium SulfateConcentration in the Feed on Once Through Conversion to SodiumBicarbonbate in Sodium Bicarbonate Precipitation Step

[0133] This example illustrates the negative impact of too much water inthe sodium sulfate feed solution on the once through conversion tosodium bicarbonate in the sodium bicarbonate precipitation step. Thecalculations were done utilizing the process simulator Hysis™ coupledwith OLI's™ property package.

[0134] Take as an example, the feed shown in Table 3 below which isderived from sodium bicarbonate scrubbing of flue gas generated byburning coal.

[0135] This feed has a water concentration of 78.1 wt % and when it ismixed with 112.2 kg of anhydrous ammonium bicarbonate (ammonium tosodium molar ratio of 1.10) and the temperature is adjusted to 38° C.,47.0 kg of sodium bicarbonate precipitate is produced. The once throughconversions of the sodium and bicarbonate to sodium bicarbonate are19.6% and 39.4% respectively. TABLE 3 EXAMPLE FEED SOLUTION COMPOSITIONComponent Flows Water - H₂O kg 717.0 Carbon Dioxide - CO₂ kg 0.0Ammonia - NH₃ kg 0.0 Sodium Ion - Na kg 65.5 Ammonium Ion - NH₄ kg 0.0Carbonate Ion - CO₃ kg 4.3 Bicarbonate Ion - HCO₃ kg 3.8 Sulphate Ion -SO₄ kg 119.0 Nitrate Ion - NO₃ kg 5.6 Fluoride Ion - F kg 0.2 ChlorideIon - Cl kg 2.5 Hydrogen Ion - H kg 0.0 Hydroxide Ion - OH kg 0.0 Totalkg 917.7 Component Wt % Water - H₂O 78.1 Carbon Dioxide - CO₂ 0.0Ammonia - NH₃ 0.0 Sodium Ion - Na 7.1 Ammonium Ion - NH₄ 0.0 CarbonateIon - CO₃ 0.5 Bicarbonate Ion - HCO₃ 0.4 Sulphate Ion - SO₄ 13.0 NitrateIon - NO₃ 0.6 Fluoride Ion - F 0.0 Chloride Ion - Cl 0.3 Hydrogen Ion -H 0.0 Hydroxide Ion - OH 0.0 Total 100.0

[0136] Removing 179.7 kg of water from this stream drops the waterconcentration to 72.8 wt % and when this concentrated stream is mixedwith 112.2 kg of anhydrous ammonium bicarbonate and the temperature isadjusted to 38° C., the “once through” production of sodium bicarbonateincreases from 47 kg to 69.3 kg. The “once through” conversions of thesodium and bicarbonate to sodium bicarbonate increase from 19.6% to29.0% and from 39.4% to 58.1% respectively.

[0137] Therefore, reducing the amount of water in the sodium sulfatefeed solution significantly increases the conversion of sodium andbicarbonate to sodium bicarbonate. It also significantly improves theoverall process efficiency since the size of the recycle streams isinversely proportional to the sodium conversion efficiency.

Example 3 Impact of Water Concentration on Salt Produced in CombinedSalt Precipitation Step

[0138] This example illustrates the negative impact of incorrect waterconcentration in the combined salt precipitation step.

[0139]FIG. 6 shows the Janecke diagram that represents the phaseequilibrium in the combined salt precipitation step at a temperature of0° C. If point A represents the charge fraction plot of the feed, itwill be obvious to one skilled in the art that it is very important toensure that the water concentration in the feed is adjusted such thatthe final mother liquor “stops” before the Glauber's salt/ammoniumbicarbonate/double salt invariant point (point B) is reached ie. atpoint C. Otherwise, double salt will form in addition to combined salton a once through basis. This means that products (ammonium sulfate)begin to recycle back to the sodium bicarbonate precipitation step,reducing the overall efficiency of the process. This contamination willget worse as the process reaches a new equilibrium deteriorating thecommercial viability of the process.

[0140] The following calculations done utilizing the process simulatorHysis™ with the OLI™ property package emphasize this point. Take as anexample, aA combined salt precipitation step feed with the compositionshown in Table 4 below is exemplified. TABLE 4 EXAMPLE COMBINED SALTPRECIPITATION STEP FEED Component Flow Water - H₂O kg 695.8 CarbonDioxide - CO₂ kg 0.3 Ammonia - NH₃ kg 1.1 Sodium Ion - Na kg 38.3Ammonium Ion - NH₄ kg 94.6 Carbonate Ion - CO₃ kg 20.7 Bicarbonate Ion -HCO₃ kg 117.5 Sulphate Ion - SO₄ kg 198.0 Nitrate Ion - NO₃ kg 5.8Fluoride Ion - F kg 0.2 Chloride Ion - Cl kg 2.6 Hydrogen Ion - H kg 0.0Hydroxide Ion - OH kg 0.0 Total kg 1174.7 Component Wt % Water - H₂O59.2 Carbon Dioxide - CO₂ 0.0 Ammonia - NH₃ 0.1 Sodium Ion - Na 3.3Ammonium Ion - NH₄ 8.1 Carbonate Ion - CO₃ 1.8 Bicarbonate Ion - HCO₃10.0 Sulphate Ion - SO₄ 16.9 Nitrate Ion - NO₃ 0.5 Fluoride Ion - F 0.0Chloride Ion - Cl 0.2 Hydrogen Ion - H 0.0 Hydroxide Ion - OH 0.0 Total100.0

[0141] To this feed, 191.3 kg of double salt recycled from thedownstream double salt precipitation step is added. If 161.0 kg of wateris also added and the mixture chilled to 0° C., the resultant saltprecipitated will contain 100 kg of ammonium bicarbonate, 217 kg ofGlauber's salt and no double salt. If the 161.0 kg of water is notadded, the resultant salt precipitated will contain 111.6 kg of ammoniumbicarbonate, 198.4 kg of Glauber's salt and 42 kg of double salt. Notonly has this increased the mass flow of the combined salt recycle by10% (on a once through basis), but there is also a product beingrecycled (ammonium sulfate) back to the sodium bicarbonate precipitationstep. If this double salt contamination is allowed to continue (by notproperly adjusting the water content), the efficiency of the processdeteriorates.

[0142] Carbon dioxide from the sodium bicarbonate crystallizer andpossibly the bicarbonate removal step or external sources is also addedto the combined salt precipitation step to push the anion chargefraction to the right. This helps in conjunction with proper wateradjustment to keep double salt from forming.

Example 4 Illustration of Chemical Equilibrium Involved in theProduction of Pure Ammonium Sulfate From a Mixed Solution of SodiumSulfate and Ammonium Sulfate

[0143]FIG. 7 shows the T-x (temperature-composition) diagram thatapplies to the chemical equilibrium involved in the production of highquality ammonium sulfate from solutions containing sodium sulfate andammonium sulfate.

[0144] An analysis of FIG. 7 reveals that a very slight change in thecation charge fraction (Y axis) of the solution can shift it from theammonium sulfate saturation plane to the sodium sulfate or double saltsaturation planes. If this happens, it is not possible to produce highquality ammonium sulfate. The prior art was deficient in demonstratingthe understanding of this system as shown in FIG. 7. This deficiencymade it very difficult to manipulate the process variables to produce asolution with a cation charge fraction that falls in the ammoniumsulfate saturation plane.

[0145] As another example, the solution with the composition as shown inTable 5 was studied. TABLE 5 EXAMPLE AMMONIUM SULFATE/SODIUM SULFATESOLUTION Component Flow Water - H₂O Kg 1127.8 Carbon Dioxide - CO₂ Kg0.0 Ammonia - NH₃ Kg 2.3 Sodium Ion - Na Kg 37.0 Ammonium Ion - NH₄ Kg190.7 Carbonate Ion - CO₃ Kg 0.0 Bicarbonate Ion - HCO₃ Kg 0.0 SulphateIon - SO₄ Kg 450.1 Nitrate Ion - NO₃ Kg 108.8 Fluoride Ion - F Kg 2.5Chloride Ion - Cl Kg 32.9 Hydrogen Ion - H Kg 0.0 Hydroxide Ion - OH Kg0.0 Total Kg 1952.0 Component Wt % Water - H₂O 57.8 Carbon Dioxide - CO₂0.0 Ammonia - NH₃ 0.1 Sodium Ion - Na 1.9 Ammonium Ion - NH₄ 9.8Carbonate Ion - CO₃ 0.0 Bicarbonate Ion - HCO₃ 0.0 Sulphate Ion - SO₄23.1 Nitrate Ion - NO₃ 5.6 Fluoride Ion - F 0.1 Chloride Ion - Cl 1.7Hydrogen Ion - H 0.0 Hydroxide Ion - OH 0.0 Total 100.0

[0146] The cation charge fraction (Y axis in FIG. 6) is calculated asfollows:

cation charge fraction=1.609/(1.609+10.594)=0.13

[0147] Referring to FIG. 6, with a cation charge fraction of 0.13, thesolution falls on the ammonium sulfate saturation plane providing thetemperature and water content are also adjusted correctly. By adjustingthe feed solution such that it falls on the ammonium sulfate saturationplane, it is possible to produce high purity ammonium sulfate with thecorrect amount of water removal or cooling. If the above solutioncontained 125 kg of sodium instead of 37 kg, the moles of sodium wouldbe 5.435 kgmoles and the cation charge fraction would be 0.34. At thiscation charge fraction, it would be impossible to produce pure ammoniumsulfate. Assuming that the temperature and water content were such thatthe solution falls onto the sodium sulfate saturation plane just abovethe sodium sulfate/ammonium sulfate co precipitation line, only sodiumsulfate would be produced until this line is hit, at which point amixture of ammonium sulfate and sodium sulfate would be produced (ifwater were removed from the system). If instead of removing water thesolution is cooled, sodium sulfate would precipitate until the sodiumsulfate/double salt saturation line is reached at which point sodiumsulfate and double salt would co-precipitate. There would be no ammoniumsulfate production at all.

[0148] The present invention elegantly ensures that the solution fromwhich pure ammonium sulfate is precipitated falls within the ammoniumsulfate saturation plane. This is accomplished by the uniqueconfiguration of the combined salt precipitation step followed by thedouble salt precipitation steps which control the amount of sodium inthe solution.

[0149] Although specific embodiments of the invention have beendescribed above, it is not limited thereto and it will be apparent tothose skilled in the art that numerous modifications form part of thepresent invention insofar as they do not depart from the spirit, natureand scope of the claimed and described invention.

We claim:
 1. A method for recovering purified sodium bicarbonate andammonium sulfate from a solution, containing sodium sulfate, comprisingthe steps of: A) providing a solution containing sodium sulfate; B)precipitating, in a single precipitation step, sodium bicarbonateprecipitate to reduce the sodium bicarbonate concentration in solution,said solution containing ammonium sulfate, said single precipitationincluding; C) removing the said sodium bicarbonate precipitate out ofsolution; D) converting in a conversion step, reactants from step B) tosodium bicarbonate and conversion step including i) adding combined saltcontaining ammonium bicarbonate and Glauber's salt to inlet sodiumsulfate solution; ii) adding carbon dioxide and ammonia gas to the saidinlet sodium sulfate solution; iii) maintaining a ammonium to sodiumratio of not less than 1; iv) operating at a temperature sufficient toprevent excessive gas production; and v) removing said sodiumbicarbonate precipitate out of solution; E) mixing said solution fromstep B) with an ammonium sulfate/sodium sulfate double salt; F) coolingsaid mixture from step E) to form a combined salt; G) precipitating saidcombined salt and removing said combined salt out of solution; H)removing residual bicarbonate from said solution from step G); I) mixingsaid solution from step H) with mother liquor; J) cooling the mixturefrom step I) to precipitate double salt; K) separating precipitateddouble salt from said solution and recycling to step E); and L)recovering ammonium sulfate from the solution of step K) byconcentrating the solution.
 2. The method as set forth in claim 1,wherein said solution of sodium sulfate is derived from a sodiumbicarbonate flue gas scrubbing operation.
 3. The method as set forth inclaim 2, wherein said solution of sodium sulfate contains contaminantsselected from the group consisting of sodium carbonate, sodium sulfite,sodium nitrate, sodium nitrite, sodium chloride and sodium fluoride. 4.The method as set forth in claim 3, wherein said contaminants are purgedfrom the process by removing a mother liquor slipstream.
 5. The methodas set forth in claim 4, where said mother liquor slipstream is derivedfrom the solution of step L).
 6. The method as set forth in claim, 1,wherein said inlet sodium sulfate solution is concentrated by removal ofwater such that the said solution contains not less than 25 wt % sodiumsalts.
 7. The method as set forth in claim 1, wherein said ammonium tosodium ratio is maintained in the optimal range of between 1.00 and1.10.
 8. The method as set forth in claim 1, wherein said ammonium tosodium ratio is maintained by the addition of ammonia gas and carbondioxide gas or ammonium bicarbonate.
 9. The method as set forth in claim1, wherein the temperature in the sodium bicarbonate precipitation stepis maintained in the optimal range of between 35 C and 40 C.
 10. Themethod as set forth in claim 1, wherein said residual bicarbonate isremoved from said solution of step G) by temperature stripping.
 11. Themethod as set forth in claim 10, wherein resultant carbon dioxide,ammonia and water is recycled to step B).
 12. The method as set forth inclaim 10, wherein resultant carbon dioxide, ammonia and water isrecycled to step E).
 13. The method as set forth in claim 10, whereinthe an overhead temperature in said said temperature stripping operationis maintained optimally between 65 C and 70 C.
 14. The method as setforth in claim 1, wherein said residual bicarbonate is removed from saidsolution of G) by acidification.
 15. The method as set forth in claim14, wherein resultant carbon dioxide is recycled to step B).
 16. Themethod as set forth in claim 14, wherein resultant carbon dioxide isrecycled to step E).
 17. The method as set forth in claim 1, whereinsaid step of concentrating (step L) comprises evaporating water fromsaid solution.
 18. The method as set forth in claim 17, furtherincluding the step of separating concentrated ammonium sulfate andresidual solution.
 19. The method as set forth in claim 1, furtherincluding the step of adding water in step E).
 20. The method as setforth in claim 1, further including the step of adding carbon dioxide instep E).
 21. The method as set forth in claim 1, further including thestep of recycling combined salt formed in step G) to step B).
 22. Themethod as set forth in claim 1 wherein carbon dioxide formed in step B)is introduced at step E).
 23. The method as set forth in claim 12,wherein said ammonia gas and said carbon dioxide gas are recycled tostep b). The method as set forth in claim 1, wherein the temperatures insteps F) and J) are maintained optimally between −2 and 2 C.
 23. Themethod as set forth in claim 1, wherein said residual bicarbonate isremoved from the solution from step K).
 24. The method as set forth inclaim 1, wherein said residual bicarbonate is removed in step L). 25.The method as set forth in claim 1 wherein, wherein solution from stepH) is processed into alternate fertilizers.